Question about Oauth2?

Question

OAuth

Hello! I recently started learning Oauth2 and decided to add password,auth code grant types to my project. Several questions arose:
1) Is grant type authorization_code used by applications when a person wants to use his account to log in to other applications? Or also for local users via user,password?
2) If locally users come in through login, password, then it is normal to use password flow?
3) I looked at examples on the Internet and they pass client_id, client_secret with basictype authorization, is it safe to do this? How to do it better? Thank you all in advance!